Piston fit with o rings

[Phil1965]

Hi I am using o rings for the first time as a piston ring on a steam engine. The cylinders and piston are in gunmetal  with a 1” bore. The drawing suggests a 1”old x1/8 profile. Any guidance what the ideal groove width and diameter would be gratefully appreciated
Thank you in advance

[Jasonb]

0.132" deep groove x 0.160 wide

[Phil1965]

Thankyou Jason. Short and simple solution 😀

[Vixen]

0.132" deep groove x 0.160 wide

Jason,

A 0.132" deep groove for the quoted 1/8" profile O ring, looks a bit too deep to me.       How did you arrive at 0.132"  ?   

But what do I know about anything?

Mike

[crueby]

A lot depends on the size o ring, there are lots of different cross section diameters. I use the viton ones that are 1/16" cross section.

[astroud]

James Walker design notes for a 0.125 cross section O ring suggest a groove diameter of cylinder bore minus  2 x 0.110 max   minus 2 x 0.114 min

[Jasonb]

Mike, an 1/8" nominal O ring as the OP says he has (BS210) actually has a Cross section of 0.139" so not too deep to give a little compression of the ring. These are the figures I use on both my IC and steam engines with Viton rings and are taken from Tubal Cain's "Model Engineers Handbook" and based on tests done by Arnold Throp. They can also be found in a Reeves paper catalogue

If it's good enough for those two then it's good enough for me

These figures compress the o ring far less than what you will see in the manufacturer's data sheets but we are not trying to hold back several hundread PSI with the resulting high friction that amount of ring compression produces.

Like Crhis I typically use a smaller section ring for my steam engines around the 1" bore. For the many 24mm bore ones I go with metric 2.4mm actual cross section rings  I use a 2.3mm deep groove and those that have built to my published drawings have also found that works fine on both <5psi air for display as well as running on steam.

[Jasonb]

Go and look up any chart for BS imperial O rings, the nominal sizes are all smaller than the actual cross section of the cord and have always been that way.

[Jasonb]

From ME Handbook

[Vixen]

Go and look up any chart for BS imperial O rings, the nominal sizes are all smaller than the actual cross section of the cord and have always been that way.

Thank you for the explanation Jason. I was not familiar with BS imperial O rings nominal sizes being smaller than the actual cross section of the cord. Something learned today.

Mike

[crueby]

For model pistons with rings, the compression of the ring that the charts give would be pretty snug, tight for an easy slide fit. Good starting point though!

[Alex]

- When I finished my 3-1/2" gauge Tich back in the '90s, I put "O" rings on piston, and piston/valve rods.

For the valve rods, I made a "D" bit to get the OD just "whisper" tight, and used a simple flat plate with screw to hold them in place.

For the pistons, first try too tight, so I put graphite packing on. One day, when it would not make it up the hills at the local club, I re-did the O-ring depth on the pistons, and it transformed the locomotive.

I have not run it in a decade or so; maybe I should pull it out and check, but I'd expect it to roll just fine.

- O-rings on my Shay locomotive pistons, etc,, and, on my "will be finished sometime" Ivatt 2-6-0, o-rings on pistons, glands and piston-valves.

- when I started out, someone (Art Ellis, if anyone in North America remembers) gave me a ME handbook about 0-rings, but the writer used too high a squeeze factor; we are not running at 10,000 psi here...

[Bobsmodels]

Hi
Just to add a bit to the suggestions.  I have used o rings in a number of engines.  You really want very low friction.  I have found that a difference between the diameter of the O-Ring and the Cylinder should be about .004" and .005".  That gives a nice seal and very little drag.   I would also recommend a 1/16 O-Ring rather than 1/8"  from a friction point of view.  For example if you have a perfect 1" bore  then you want the groove diameter to be 1.00" -( 2 x .070 (actual diameter of a 1/16 O-ring) - .005) = .865 diameter.  Note does not mater what diameter your piston is, the groove diameter is the same as it is based off of the cylinder bore.  Also minimal roll for width, I use .082 for a 1/16 ring.   Attached is a sketch showing arbitrary piston diameter could be any reasonable size, and PDF showing how I did it for my Gade.  Here is a link to a video showing low friction with my Gade, which had about 600 hours on it at the time with the same o-ring, vitron.

 <a href="https://www.youtube.com/watch?v=1QazAr4b3KY" target="_blank">http://www.youtube.com/watch?v=1QazAr4b3KY</a>

Bob